how to export and import realm data in android - export

Question

I know there are those questions which are similar to mine have already raised.
However, I can not go further more on some point.
** I can not copy file on below.-> realm.writeCopyTo(exportRealmFile);
** The code that I referred to is below.
https://stackoverflow.com/a/36324183/6650123
And let me share my application class which I defaulted Realm.
Wondering if the realm file can not be copied because of that..
Would you take a look and advise?
**
public class MyApplication extends Application {
private static MyApplication singleton;
private RealmConfiguration realmConfig;
String TAG=getClass().getName();
public static MyApplication getSingleton(){;
return singleton;
}
public void onCreate() {
super.onCreate();
singleton=this;
// Create the Realm configuration
realmConfig = new RealmConfiguration.Builder(this).build();
Realm.setDefaultConfiguration(realmConfig);
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
}
public void onLowMemory() {
super.onLowMemory();
}
public void onTerminate() {
super.onTerminate();
}
}
// my codes for backup realm
package com.first.project;
import android.content.Context;
import android.os.Environment;
import android.util.Log;
import android.widget.Toast;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import io.realm.Realm;
public class RealmBackupRestore {
private final static String TAG = RealmBackupRestore.class.getName();
private Context context;
private Realm realm;
public RealmBackupRestore(Context context) {
realm= Realm.getDefaultInstance();
//this.realm = Realm.getInstance(BaseApplication.realmConfiguration);
this.context = context;
}
public void backup() {
Log.d(TAG, "realm은"+realm);
Log.d(TAG, "Realm.getDefaultInstance();은"+Realm.getDefaultInstance());
File exportRealmFile = null;
File exportRealmPATH = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);
Log.d(TAG, "exportRealmPATH"+exportRealmPATH);
String exportRealmFileName = "aa.realm";
Log.d(TAG, "Realm DB Path = " + realm.getPath());
try {
// create a backup file
exportRealmFile = new File(exportRealmPATH, exportRealmFileName);
// if backup file already exists, delete it
exportRealmFile.delete();
// copy current realm to backup file
realm.writeCopyTo(exportRealmFile);
} catch (IOException e) {
e.printStackTrace();
}
String msg = "File exported to Path: " +Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);
Toast.makeText(context, msg, Toast.LENGTH_LONG).show();
Log.d(TAG, msg);
realm.close();
}
public void restore() {
//Restore
File exportRealmPATH = context.getExternalFilesDir(null);
String FileName = "default.realm";
String restoreFilePath = context.getExternalFilesDir(null) + "/" + FileName;
Log.d(TAG, "oldFilePath = " + restoreFilePath);
copyBundledRealmFile(restoreFilePath, FileName);
Log.d(TAG, "Data restore is done");
}
private String copyBundledRealmFile(String oldFilePath, String outFileName) {
try {
File file = new File(context.getFilesDir(), outFileName);
Log.d(TAG, "context.getFilesDir() = " + context.getFilesDir().toString());
FileOutputStream outputStream = new FileOutputStream(file);
FileInputStream inputStream = new FileInputStream(new File(oldFilePath));
byte[] buf = new byte[1024];
int bytesRead;
while ((bytesRead = inputStream.read(buf)) > 0) {
outputStream.write(buf, 0, bytesRead);
}
outputStream.close();
return file.getAbsolutePath();
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
private String dbPath() {
return realm.getPath();
}
}